Broken points of optical fiber cables, distributions of transmission losses, coupling losses, etc. can be measured by an optical pulse testing apparatus (OTDR; Optical Time Domain Reflectometer).
The structure of a conventional optical pulse testing apparatus will be explained with reference to FIG. 8. FIG. 8 is a block diagram of the structure of the optical pulse testing apparatus.
The conventional optical pulse testing apparatus includes a semiconductor laser 100 as a means for generating optical pulses as a probe. The semiconductor laser 100 is connected to a pulse generating circuit 102 which operates so that the semiconductor laser 100 outputs optical pulses at a predetermined cycle. The semiconductor laser 100 has the output connected to an optical fiber-to-be-measured 104 via a directional coupler 108 and an input/output connector 106. The directional coupler 108 is connected to a photodetector 110 which detects returning light reflected from the optical fiber 104. The photodetector 110 is connected to an amplifier 112 which amplifies electric signals from the photodetector 110. The amplifier 112 is connected to an A-D converter 114 which converts electric signals amplified by the amplifier 112 to digital signals. The A-D converter 114 is connected to a computer circuit 116 which computes digital signals supplied by the A-D converter 114. The computer circuit 116 includes a display 118 which indicates measured results given by the computer circuit 116. The A-D converter 114 and the computer circuit 116 are connected to a timing circuit 120 to be controlled by the timing circuit 120.
Then, the principle of the measuring of the optical pulse testing apparatus will be explained with reference to FIG. 8.
First, the timing circuit 120 inputs trigger signals to the pulse generating circuit 102. In response to the trigger signals, the pulse generating circuit 102 generates pulse currents for driving the semiconductor laser 100. The semiconductor laser 100 is controlled by the pulse currents from the pulse generating circuit 102 to output optical pulses of a predetermined pulse width and a predetermined cycle. Optical pulses emitted by the semiconductor laser 100 are supplied to the optical fiber-to-be-measured 104 connected to the input/output connector 106 via the directional coupler 108.
Optical pulses incident on the optical fiber-to-be-measured 104 propagate in the optical fiber-to-be-measured 104. Reflected light due to mismatching of the transmission paths at the coupling points of the optical fiber-to-be-measured 104, back scattering light, such as Rayleigh scattering light caused by trivial a small amount of disuniformity in the optical fiber-to-be-measured 104 returns to the optical pulse testing apparatus. A time from the incidence of an optical pulse on the optical fiber-to-be-measured 104 till the return of the optical pulse to the optical pulse testing apparatus is proportional to a distance from the end of the optical fiber-to-be-measured 104 into which the optical pulse was introduced to a reflection point or a scattering point of the optical fiber 104.
The light which has returned from the input/output connector 106 to the optical pulse testing apparatus is detected by the photodetector 110 via the directional coupler 108 and converted to electric signals. The converted electric signal is amplified by the amplifier 112, then converted to digital signals by the A-D converter 114, based on signals from the timing circuit 120, and synchronously added at each cycle of the pulses by the computing circuit 116. Light propagating through an optical fiber exponentially attenuates. Therefore, the added signals are logarithmically transformed by the computer circuit 116. The measured results are presented on the display 118 in distances of the optical fiber-to-be-measured 104 proportional to periods of time on the horizontal axis and intensities of the reflected light or the scattering light on the vertical axis. Broken points of an optical fiber and loss distributions in the optical fiber can be thus measured.
In the above-described optical pulse testing apparatus, the means for generating pulses as the probe can be provided by a pulse excitation variable wavelength ring laser in place of the semiconductor laser 100.
An example of structure in such a pulse excitation variable wavelength ring laser is shown in FIG. 9. As shown in FIG. 9, the pulse excitation variable wavelength ring laser includes an optical fiber 122 doped with rare earth element (s) for photoamplification.
The optical fiber 122 has an excitation light source 124 which is disposed via an optical multiplexer 126 for exciting the optical fiber 122. The excitation light source 124 is connected to a light source driving circuit 134 to drive the excitation light source 124 at selected excitation intensities, selected time intervals and selected repetition frequencies.
In the variable wavelength ring laser of the structure shown in FIG. 9, after the excitation light source 124 is turned on by the light source driving circuit 134, the excitation of the optical fiber 122 is started; laser outputs are not obtained until a period of time in which the laser oscillation starts, and becomes CW (continuous wave) outputs of the CW excitation ring laser after a plurality of optical pulse series are temporarily generated during the transient period. FIG. 10A shows outputs of the variable wavelength ring laser whose excitation time of the excitation light source 124 is shortened and is optimized to produce single pulse outputs.
Structures of such variable wavelength ring laser which can produce single pulse outputs have been proposed. One example of the conventional variable wavelength ring lasers which can easily produce single pulse outputs will be explained with reference to FIG. 11.
The variable wavelength ring laser shown in FIG. 11 includes an optical switch 136 disposed between the optical branching filter 130 and the isolator 132 in addition to the structure of the variable wavelength ring laser shown in FIG. 9. The optical switch 136 is connected to an optical switch controller 138 for controlling the optical switch 136.
The optical switch 136 is turned on for a short period of time by the optical switch controller 138 and can generate optical pulses of high output power corresponding to a length of the optical fiber 122 and a concentration level of rare earth element(s) added (doped) to the optical fiber 122. Single pulses as the outputs are taken out by the optical branching filter 130. The details are described in Japanese Patent Laid-Open Publication No. Hei 5-21880 (1993).
However, parameters, such as a peak intensity, a half-value width, a delay time, etc. of a single pulse output generated by the conventional pulse excitation variable wavelength ring laser shown in FIG. 9 are determined by a concentration ratio of rare earth element(s) added to the optical fiber 122, a length of the optical fiber 122, an excitation intensity of the excitation light source 124, a repetition frequency of an excitation pulse, an excitation pulse width, a branch ratio of the optical branching filter 130, an oscillation wavelength determined by the variable wavelength filter 128 and an overall length of the ring laser constituted by the above noted optical parts. Accordingly, an excitation intensity and an excitation pulse width of the excitation light source 124 must be controlled corresponding to intended wavelength, output and a repetition frequency, and a timing control of the optical pulse testing apparatus must be adjusted.
On the other hand, the use of the variable wavelength ring laser shown in FIG. 11 to simplify the timing control of the optical pulse testing apparatus requires the expensive optical switch 136 for generating optical pulses.